New York — New YorkOn the ground, New York City seems to be an unparalleled exercise in modern convenience. The city is home to the only subway in America that runs 24 hours a day. New Yorkers can get most anything – from household groceries ordered on the Internet to Sichuan food – delivered almost anywhere almost any day of the year. New Yorkers are limited in what can be bought, sold, stored, or serviced only by their own imaginations.

However slick the city’s neon-lit streets, what keeps the city pumping – literally – towers high above the bustling crowds.

On top of every building more than six stories high, from the Bronx to Brooklyn, there’s a water tank, a rotund wooden barrel that holds the thousands of gallons of water apartment dwellers need for cooking, showering, and washing clothes.

“Once you tell somebody what you do for a living, they say, ‘Holy mackerel, I look out of the office at work, and they’re all over the place!’ ” says Kenny Lewis, shop foreman of the Rosenwach Tank Company’s lumber yard. “Yes, they are. And thank God, because that’s what we do.”

In fact, hardly a day goes by when owner Andrew Rosenwach doesn’t get a call, from a new apartment complex in Queens to Cartier Jewelers’ New York City flagship, to repair or restore a tank his father or grandfather installed years before.

Rosenwach is one of only two companies in town that make the wooden water towers; a discerning survey of the skyline can reveal whose towers are whose: A little “R” rests against each side of the post that pokes out from the top of a Rosenwach tank, like a wooden flag.

“I was getting out of my car, and two men stopped me and ... [said], ‘You can’t be a New Yorker without knowing about Rosenwach,’ ” says Mr. Rosenwach. “Another guy gave me a bear hug and said, ‘You’re the the Gunga Din of New York.’ ”

Rosenwach lacks the “squidgy nose” of the water-carrying hero of the Rudyard Kipling poem, but the comparison is one of Rosenwach’s favorites. The fourth in the family to run the 112-year-old company, he was a lover of literature first, and a tank man only with time. He studied liberal arts at Johns Hopkins, buddying up with late literary critic Hugh Kenner, and worked at his father’s company in the summers, mostly sweeping up debris and hoisting planks up to the men building the tanks.

“I never really did go up on top. I basically would’ve fallen or tripped on my shoelaces or something,” he says. “You really have to have strong legs, know how to balance yourself, and know how to move.... It’s like being a performer ... 50 feet off the roof.”

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Rosenwach is a fairly short man, always in a leather Sunday driver’s cap and always tethered to his cellphone. He runs a business that is, in many ways, an anachronism. In a city familiar with the multimillion-dollar merger, he is at the helm of an enterprise that’s been in his family since his great-grandfather, Harris Rosenwach, bought out his boss’s barrelmaking company for $55 in 1896. Barrelmakers are still known as coopers, though it’s a word few today use, and fewer still know is a skill originally taught to white New Yorkers by their slaves.

But little a cooper does has changed in the centuries the trade has been around, and Rosenwach’s tanks are made the same way today as they were a century ago: no glue, no sealants, no linings. The wood simply soaks up some of the water it holds, swells, and seals itself. All a Rosenwach tank needs are a few strategically placed nails and a set of steel hoops that wrap like belts around the tank’s body, holding it in place while the water is released.

For all of its cosmopolitan glamour, you could say that when it comes to basic hygiene, New York City is stuck, without realizing it, in the 19th century. Historians say that most of the plumbing was laid beneath the city grid in the 1840s; one generation later, after the Civil War, the city’s population exploded. Tenements went up five and six stories high, but the water pressure couldn’t keep top floors supplied all the time. Enter the water tank: In one big chug, thousands of gallons of water were pumped up to the roof, where they’re collected in a tank.

Today, it’s still the method used for water delivery – even in skyscrapers. When building occupants need water to boil rice, flush toilets, or take candle-lit baths, the water runs from the roof to the tap, pulled down by gravity.Maybe it’s the simple physics behind their utility, or maybe it’s their sheer ubiquity, but the tanks have become symbols of the city. The New York Historical Society commissioned Rosenwach to build a tank indoors as part of an exhibit on the rooftops of New York City. The Museum of Modern Art erected a plastic replica of a tank on its roof. Many an art class has been hauled to the roof to sketch the icons of the skyline; at a good tourist shop, you might find magnets, or penny banks, or other paraphernalia in the shape of a wood tank. In the 1970s, the Washington Zoo asked Rosenwach to build two redwood tanks as bathtubs for its pandas, and a tank aficionado in Windsor, N.Y., had the company build him a tank that he then lived in.

So there may be something more than marketing to Rosenwach’s favorite chorus. “Wood tanks,” he likes to say, “make people happy.”

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“Let me show you the tree that grows in Brooklyn,” Rosenwach says in his lumber yard. Once a horse stable, the lot is full of planks that will be tanks. Nestled behind a stack of them is a pine tree, its roots bursting from a barrel. The tree was given to the family years ago and set outside in the barrel. To hear Rosenwach describe it, it sounds like the safest tree in a lumber yard anywhere in America: “It’s so endearing, we could never leave this property because the tree is like family.”

But this is still America, and business is always busniess. As he leaves the tree that grows in Brooklyn, Rosenwach jokes, “if it gets any bigger I may have to make a tank out of it.”